While metal–organic frameworks have been extensively investigated for gas storage and separation, their thermal properties are scarcely studied and largely unknown. We report here a molecular ...simulation study to examine the thermal conductivity of zeolitic imidazolate framework-8 (ZIF-8). From equilibrium molecular dynamics simulation, the thermal conductivity of ZIF-8 is predicted to be about 0.165 W/mK. The low thermal conductivity is due to the short mean free path of the phonon in ZIF-8, which is estimated to be less than two unit cells. With increasing temperature from 300 to 1000 K, the thermal conductivity increases from 0.165 to 0.190 W/mK. The temperature effect is attributed to the enhanced overlap in the vibrational density of states between Zn and N atoms. The contributions of lattice vibrations in different directions to heat flux are examined from nonequilibrium molecular dynamics simulation. It is found that the longitudinal vibration contributes 60% to thermal transport in ZIF-8; in contrast, transverse vibration contributes 40%. Furthermore, the contributions from different forces to heat flux are analyzed. While the stretching and Lennard-Jones components have 31% contribution each, the bending and Coulombic components contribute 17% and 21%, respectively. However, the contribution of the torsional component is nearly zero. This simulation study provides quantitative understanding of the thermal conductivity of ZIF-8 and microscopic insight into the mechanism of heat transfer.
Much attention is currently paid to microplastic (MP) pollution, particularly in marine systems. There is increasing concern regarding the potential toxicity of MPs to organisms at the physiological ...and morphological levels. However, little is known about the impact of MPs on aquatic life, despite their ubiquitous presence in freshwater ecosystems. In this study, the aquatic plant Utricularia vulgaris was exposed to 1, 2 and 5 μm polystyrene fluorescent MP particles at concentrations of 15, 70 and 140 mg/L for 7 days. The toxic effects of MPs on the growth rate and morphological and physiological characteristics of U. vulgaris were assessed. The results showed that the relative growth rates and the functional traits of leaves (morphological and photosynthetic) were significantly inhibited at a high concentration of MP particles (140 mg/L) when compared to the control group. The impacts on growth performance were likely due to bioaccumulation of MPs in the bladders, as shown by confocal microscopy. Furthermore, the antioxidative enzyme activities showed that high concentrations of MPs induce high ecotoxicity and oxidative damage to U. vulgaris. Thus, U. vulgaris has the potential to be an excellent bioindicator of MP pollution in freshwater ecosystems and should further be applied in ecological risk assessments of the effects of MPs on higher aquatic plants.
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•We explored effects of microplastics for submerged carnivorous aquatic plants.•Polystyrene microplastics have been ingested by the bladders.•Growth performance of U.vulgaris was reduced with increasing microplastic concentration.•U. vulgaris can be potential eco-friendly purifiers for microplastics biodegradation.
To promote the development level of urban sustainability, more and more cities have been paying attention to the improvement of public transportation. City managers intend to attract people from ...private cars to public transport by improving the service level of urban public transport. The operational reliability of bus lines plays a crucial role in maintaining high-level service of public transportation. Previous studies have focused on the service level of the whole line by investigating the overall stability of departure punctuality, line running time, and punctuality. This study aims to clarify the relationship between the whole line and sites in terms of the influence on the operational reliability of bus lines. This study proposes a bus route reliability evaluation method, based on copulas connect function, and the actual line data are taken as a case study. The results show that the method reveals the relationship between the overall line and the interstation paths in the context of operational reliability. This study finds the interstation path, which is more critical to the whole line. This method can provide a good reference for the diagnosis and evaluation of line operation and the optimization of bus routes. The results can support the sustainable development of public transportation.
This study aimed to examine the macrophage phenotype and its relationship to renal function and histological changes in human DN and the effect of TREM-1 on high-glucose-induced macrophage ...activation. We observed that in renal tissue biopsies, the expression of CD68 and M1 was apparent in the glomeruli and interstitium, while accumulation of M2 and TREM-1 was primarily observed in the interstitium. The numbers of CD68, M1, and M2 macrophages infiltrating in the DN group were increased in a process-dependent manner compared with the control group, and the intensities of the infiltrates were proportional to the rate of subsequent decline in renal function. M1 macrophages were recruited into the kidney at an early stage (I+IIa) of DN. The M1-to-M2 macrophage ratio peaked at this time, whereas M2 macrophages predominated at later time points (III) when the percentage of M1/M2 macrophages was at its lowest level. In an in vitro study, we showed that under high glucose conditions, macrophages began to up-regulate their expression of TREM-1, M1, and marker iNOS and decreased the M2 marker MR. However, the above effects of high-glucose were abolished when TREM-1 expression was inhibited by TREM-1 siRNA. In conclusion, our study demonstrated that there was a positive correlation between the M1/M2 activation state and the progress of DN, and TREM-1 played an important role in high-glucose-induced macrophage phenotype transformation.
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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
By carefully and systematically performing Green–Kubo equilibrium molecular dynamics simulations, we report that the thermal conductivity (κ) of Si nanowires (NWs) does not diverge but converges and ...increases steeply when NW diameter (D) becomes extremely small (dκ/dD < 0), a long debate of one-dimensional heat conduction in history. The κ of the thinnest possible Si NWs reaches a superhigh level that is as large as more than 1 order of magnitude higher than its bulk counterpart. The abnormality is explained in terms of the dominant normal (N) process (energy and momentum conservation) of low frequency acoustic phonons that induces hydrodynamic phonon flow in the Si NWs without being scattered. With D increasing, the downward shift of optical phonons triggers strong Umklapp (U) scattering with acoustic phonons and attenuates the N process, leading to the regime of phonon boundary scattering (dκ/dD < 0). The two competing mechanisms result in nonmonotonic diameter dependence of κ with minima at critical diameter of 2–3 nm. Our results unambiguously demonstrate the converged κ and the clear trend of κ ∼ D for extremely thin Si NWs by fully elucidating the competition between the hydrodynamic phonon flow and phonon boundary scattering.
In order to be environment-friendly, relieve traffic congestion, reduce pollution, and be green and sustainable, the optimization and development of public transportation, as the subject of people's ...long-term research, has always been shining. With the emergence of shared transportation, public transportation systems face more challenges. In order to better connect with bike-sharing, car-sharing, and other modes of transportation, public transportation will carry out important reforms, among which the optimization of line network is one of the most important tasks. The traditional bus route design is mainly based on the “four-stage” method model, which is mainly based on the investigation and analysis of the existing traffic system and land use. Through the work flow of “evaluation, calibration, and verification,” the network balance optimization model is used to get the bus travel allocation prediction model. In this paper, the optimization problem of public transit network is studied from the point of view of the reliability of public transit network. It is proposed that public transit network can be abstracted into series-parallel system and parallel-series system model from the three states of normal, short-circuit failure, and open-circuit failure and is analyzed and discussed through the hypothesis experiment. The research of this paper will provide a new perspective for the optimization of public transit network, complement the traditional methods, and support the optimization and reliability improvement of urban public transit network. More reliable bus networks and other modes of transportation, such as walking, bike-sharing, and rail, will become more suitable for people to get around.
Silicene, the silicon-based counterpart of graphene, has received exceptional attention from a wide community of scientists and engineers in addition to graphene, due to its unique and fascinating ...physical and chemical properties. Recently, the thermal transport of the atomic thin Si layer, critical to various applications in nanoelectronics, has been studied; however, to date, the substrate effect has not been investigated. In this paper, we present our nonequilibrium molecular dynamics studies on the phonon transport of silicene supported on different substrates. A counter-intuitive phenomenon, in which the thermal conductivity of silicene can be either enhanced or suppressed by changing the surface crystal plane of the substrate, has been observed. This phenomenon is fundamentally different from the general understanding of supported graphene, a representative two-dimensional material, in which the substrate always has a negative effect on the phonon transport of graphene. By performing phonon polarization and spectral energy density analysis, we explain the underlying physics of the new phenomenon in terms of the different impacts on the dominant phonons in the thermal transport of silicene induced by the substrate: the dramatic increase in the thermal conductivity of silicene supported on the 6H-SiC substrate is due to the augmented lifetime of the majority of the acoustic phonons, while the significant decrease in the thermal conductivity of silicene supported on the 3C-SiC substrate results from the reduction in the lifetime of almost the entire phonon spectrum. Our results suggest that, by choosing different substrates, the thermal conductivity of silicene can be largely tuned, which paves the way for manipulating the thermal transport properties of silicene for future emerging applications.
Thermal conductivity of silicene can be either enhanced or suppressed by changing the surface crystal plane of the substrate, which is fundamentally different from the general understanding of supported graphene.
Phase-change materials (crystalline at low temperatures and partial-crystalline partial-liquid state at high temperatures) are widely used as thermoelectric converters and battery electrodes. Here, ...we report the underlying mechanisms driving the thermal transport of the liquid component, and the thermal conductivity contributions from phonons, vibrations with extremely short mean free path, liquid and lattice-liquid interactions in phase-changed Li
S. In the crystalline state (T ≤ 1000 K), the temperature dependent thermal conductivity manifests two different behaviors, i.e., a typical trend of 1/T below 800 K and an even faster decrease between 800 and 1000 K. For the partial-crystalline partial-liquid Li
S when T ≥ 1100 K, the contributions of liquid and lattice-liquid interactions increase significantly due to the fluidization of Li ions, and the vibrations with extremely short mean free path, presumably assimilated to diffusons, can contribute up to 46% of the total thermal conductivity at T = 1300 K.
Pursuing extremely low interfacial thermal resistance has long been the task of many researchers in the area of nano-scale heat transfer, in particular pertaining to improve heat dissipation ...performance in electronic cooling. While it is well known and documented that confining a macroscopic third layer between two dissimilar materials usually increases the overall interfacial thermal resistance, no research has realized the fundamental decrease in resistance so far. By performing nonequilibrium molecular dynamics simulations, we report that the overall interfacial thermal resistance can be reduced by 6 fold by confining mass graded materials with thickness of the order of nanometers. As comparison we also studied the thermal transport across the perfectly abrupt interface and the widely used alloyed (rough) interface, which shows an opposing and significantly large increase in the overall thermal resistance. With the help of frequency dependent interfacial thermal conductance and wave packet dynamics simulation, different mechanisms governing the heat transfer across these three types of interfaces are identified. It is found that for the rough interface there are two different regimes of interfacial heat transfer, which originates from the competition between phonon scattering and the thickness of the interface. The mechanism of dramatically improved interfacial heat transfer across the nano-confined mass graded interface resides in the minor phonon reflection when the phonons first reach the mass graded area and the rare occurrence of phonon scattering in the subsequent interior region. The phonons are found to be gradually truncated by the geometric interfaces and can travel through the mass graded layer with a high transmission coefficient, benefited from the small mass mismatch between two neighboring layers in the interfacial region. Our findings provide deep insight into the phonon transport across nano-confined mass graded layers and also offer significant guidance for designing advanced thermal interface materials.
The nano-confined mass graded interface structure is found to reduce the overall interfacial thermal resistance by 6 fold as compared with the perfectly abrupt interface. The results imply that such structure can be an excellent candidate in the electronic cooling.
It is well known that carbon nanotubes (CNTs) possess ultrahigh thermal conductivity that is comparable to bulk diamond. However, no research has studied the possible low thermal conductivity of ...different CNTs so far. By performing nonequilibrium molecular dynamic simulations, we reveal that the perfect graphyne nanotube (GNT) exhibits an unprecedentedly low thermal conductivity (below 10 W/mK at room temperature), which is generally two orders of magnitude lower than that of ordinary CNTs and even lower than the values reported for defected, doped, and chemically functionalized CNTs. By performing phonon polarization and spectral energy density analysis, we observe that the ultralow thermal conductivity stems from the unique atomic structure of the GNT, consisting of the weak acetylenic linkage (sp C-C bonds) and the strong hexagonal ring (sp super(2) C-C bonds), which results in a large vibrational mismatch between these two components, and thus induces significantly inefficient heat transfer. Moreover, the thermal transport in GNT with a large number of acetylenic linkages is dominated by the low frequency longitudinal modes in the linkage. Such strong confinement of the low frequency thermal energy results in the extremely low thermal conductivity due to the flattened phonon dispersion curves (low phonon group velocities). The exploration of the abnormal thermal transport of GNTs paves the way for design and application of the relevant devices that could benefit from the ultralow thermal conductivity, such as thermoelectrics for energy conversion.
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